1. Field of the Invention
The present invention relates to a mobile communications system, and more particularly to a control technique for the mobile communications system so as to keep system throughput at a proper level.
2. Description of the Related Art
With the recent widespread of communication terminals, such as mobile telephone terminals and personal computers, multiple-access techniques have been developed to efficiently multiplex radio signals transmitted by a plurality of user terminals located at multiple geographic points. As known well, CDMA (Code Division Multiple Access) is one of the multiple-access techniques. In the CDMA system, high-speed bidirectional transmission power control is performed at all times between a mobile terminal and a base station so that the predetermined reception quality can be ensured at one party by optimizing the transmission power of the other party.
Japanese Patent Application Unexamined Publication No. 2000-91982 discloses a technique of avoiding communication quality degradation in the CDMA mobile radio communications system. More specifically, a base station monitors the transmission information of each mobile station at all times. When detecting a failed mobile station transmitting abnormal output, the base station transmits a transmission stop instruction to the failed mobile station and checks whether an acknowledgement is received from the failed mobile station. If no acknowledgement is received, the base station forcefully stops transmitting radio waves to the failed mobile station.
However, in a case where a malfunction or degradation of receiving performance occurs in some mobile terminals of the CDMA system, such a mobile terminal cannot ensure the preset reception quality, therefore continuing to transmit a request for increasing the transmission power of a downlink signal transmitted from a base station. As a result, the transmission power of a downlink signal transmitted from the base station to these mobile terminals becomes greater than necessary. Such power-increased radio waves may adversely affect communications of other mobile terminals as interference waves.
Japanese Patent Application Unexamined Publication No. 2003-18068 discloses a CDMA system which has functions of: detecting a mobile terminal with a malfunction or degradation of receiving performance; and controlling such that the transmission power of a downlink signal to such a mobile terminal is not increased to avoid interference with communications of other mobile terminals. More specifically, the base station notifies a base station controller of the transmission power level of a downlink signal from the base station to a mobile terminal. The base station controller determines that the mobile station is out of order when the transmission power level exceeds a predetermined threshold for a predetermined time period. Such a failed mobile terminal is registered into a mobile terminal management database. The base station controller, when receiving a call set-up request from the failed mobile station, sets the failed mobile station to a target communication quality level lower than normal. If the failed mobile station continues to transmit a transmission power increasing request even in the case of the target communication quality level, then the base station controller rejects the call setup request received from the failed mobile station.
As described above, the conventional mobile communications systems are capable of detecting system failure due to a malfunction of a mobile terminal. However, there are cases where interference power increases due to not only the failure of a mobile terminal but also failures or the like in a base station.
With downsizing and cost-reduction of a radio base station in a mobile telephone system, a small-size radio base station has been developed for home use. Accordingly, an ordinary user who is not a mobile telephone carrier can possess a radio base station and may modify or adjust it without authorization or permission. Especially, modifications of a transmission power amplifier to increase the transmission power of radio signals may be relatively easy, compared with a base band signal processor, and probably bring much merit to its user. Therefore, concerns are rising that such unauthorized modifications become widespread.
Further, compared with radio base stations managed by the common carrier, a home-use radio base station may be used sloppily and therefore becomes susceptible to failure. If a failed radio base station has been used, it is considered that the transmission power of the failed radio base station becomes greater than the prescribed level. The increased transmission power reduces the radio-channel error rate of the mobile terminals under the radio base station and improves their throughput. However, as described before, such power-increased radio waves may adversely affect communications of other mobile terminals under another radio base station as interference waves. Especially, a mobile communications system such as a CDMA telephone system uses the same frequency among radio base stations and therefore the power-increased radio waves have a very large influence on other communications.
In the case of CDMA mobile communications system, the possible number of users accommodated in the system and its throughput are dependent on the ratio of signal power to interference power. Accordingly, as the transmission power of each user or base station becomes larger, it causes interfering with the communications of other users or base stations.
When the interference with the communications of the radio base station and mobile terminals accommodated therein is becoming larger, their transmission power controllers increase their transmission power so as to reduce the influence of the interference. This increased transmission power in turn affects the communications of other radio base stations and mobile terminals accommodated therein. Such transmission power increasing control is successively repeated, which increases the total power consumption in the system and reduces the possible number of users accommodated in the system and the system throughput.